A laying vessel is a floating unit, which comprises a pipeline assembly line, and is equipped at the stern with a laying ramp, which is an extension of the assembly line and serves to ease the pipeline onto the bed of the body of water as the floating unit moves forward.
Laying vessels include S-lay vessels of the above type, as described for example in Patent Application WO 2009/098586 A2, and J-lay vessels of the type described in Patent EP 1,102,698 B1.
In the S-lay method, the pipeline is assembled on a substantially horizontal assembly line and laid off the laying ramp, which, in the work configuration, serves to guide and support the pipeline along a curved path partly above and partly below water level. Pipelines laid this way assume an S shape between the laying vessel and the bed of the body of water, with curves of a radius depending on the rigidity of the pipeline, and can be laid using various types of vessels, such as pontoons, barges, semisubmersibles, and single-hull ships.
Underwater pipeline laying calls for fairly calm water conditions, so much so that, in rough water, laying is suspended, the completed part of the pipeline is abandoned using a winch-controlled cable, and the laying ramp is raised as far as possible above water level into a so-called ‘survival configuration’.
The laying ramp normally comprises an inner ramp hinged to the laying vessel, and an outer ramp hinged to the inner ramp, and can assume various work configurations, depending on the depth of the bed and the characteristics of the pipeline; a transit configuration, in which the inner and outer ramps allow unimpeded movement of the laying vessel; and said survival configuration. For which purpose, the laying ramp is connected to the laying vessel by a known actuator assembly, as described for example in the Applicants U.S. Pat. No. 4,257,718.
To guide and move the pipeline forward, the laying ramp comprises a number of guide devices aligned to define the path of the pipeline.
In the J-lay method, the pipeline assembly line is housed at least partly in a J-lay tower hinged to the laying vessel, and the pipeline is guided by a laying ramp normally fixed to the J-lay tower. In this case, too, as described in Patent EP 1,102,698 B1, adjustable guide devices are fitted to the laying ramp supporting structure to guide the pipeline as it is being laid, and to monitor stress between the pipeline and the supporting structure.
In both the above laying methods, stress between the pipeline and the laying ramp supporting structure must be monitored to prevent damage to the pipeline and/or laying ramp. In this connection, it is important to remember that, when laying the pipeline, the laying vessel and ramp, on the one hand, and the pipeline, on the other, are subjected by the water to various forms of stress. The laying vessel, despite being maintained in position and advanced in steps along a given course by a system of mooring lines or a so-called dynamic positioning system comprising thrusters, is still subject to undesired movements caused by waves and water currents. The same wave action and currents also affect the pipeline, but, because of the difference in shape and mass of the laying vessel and the pipeline, and the fact that the pipeline is substantially flexible and rests partly on the bed, produce different movements of the laying ramp and pipeline. Pitching and rolling of the laying vessel are especially hazardous, by causing violent contact between the pipeline and the laying ramp; and known systems of monitoring stress between the pipeline and the laying ramp supporting structure have proved poorly effective in preventing critical pipeline and laying ramp operating conditions.